This is a multicenter, open-label study to study the safety and characteristics of DNL310, an investigational drug designed to treat both the body and brain manifestations of Mucopolysaccharidosis type II (MPS II; Hunter syndrome). The plan is for each participant to start at a low dose and increase over time until a final dose is achieved. Blood, urine and CSF samples will be taken, and tests and assessments will be done to see how your child is doing in the study.

Hunter Syndrome or Mucopolysaccharidosis II (MPS II) is a rare genetic disorder caused by decreased levels of a specific enzyme, iduronate-2-sulfatase. Lack of this enzyme causes buildup of large sugar molecules called glycosaminoglycans (GAGs). The build up of GAGs causes significant damage to the body and brain. This study is evaluating the use of JR-141, an enzyme replacement therapy, to provide the missing enzyme to the body. This allows for break down of the stored GAGs. JR-141 uses specific technology to cross the blood brain barrier so that the enzyme can get into the brain. JR-141 is expected to help both the cognitive (brain) and body effects of MPS II.

Hunter Syndrome or Mucopolysaccharidosis II (MPS II) is a rare genetic disorder caused by decreased levels of a specific enzyme, iduronate-2-sulfatase. Lack of this enzyme causes buildup of large sugar molecules called glycosaminoglycans (GAGs). The build up of GAGs causes significant damage to the body and brain. Idursulfase-IT, an enzyme replacement therapy, was developed for the treatment of patients with Hunter Syndrome with cognitive impairment to help with the central nervous system manifestations of the disease. The purpose of this post-trial access program is so that patients with Hunter syndrome who previously received monthly administration of idursulfase-IT through participation in an earlier clinical research study can continue to receive it.

Sanfilippo syndrome Type A or mucopolysaccharidosis type IIIA (MPS IIIA) is a rare genetic disorder caused by the deficiency of the lysosomal enzyme sulfamidase. Lack of this enzyme causes buildup of large sugar molecules called glycosaminoglycans (GAGs) in the lysosomes which leads to cellular dysfunction. The buildup of GAGs causes significant damage to the body and brain. This study is a multicenter, Phase 1/2 study to assess the safety, tolerability, and efficacy of they study drug DNL126, an enzyme replacement therapy, to provide the missing enzyme to the body. Up to 28 participants with MPS IIIA will participate in the study across multiple study sites .

Mucopolysaccharidosis II (MPS II) or Hunter syndrome, is a rare genetic disorder caused by the decreased levels of a specific enzyme, iduoronate-2-sulfatase. Lack of this enzyme causes buildup of large sugar molecules called glycosaminoglycans (GAGs). The build up of GAGs causes significant damage to the body and brain. This study will evaluate the long-term use of DNL310, an enzyme replacement therapy, to provide the missing enzyme to the body and to break down the stored GAGs. DNL310 uses specific technology to cross the blood brain barrier so that the enzyme can get into the brain. DNL310 is expected to help both the cognitive (brain) and body effects of MPS II. This is an extension study to two current DNL310 clinical trials; only participants currently enrolled in these trials will have the opportunity to participate in this long-term study.